The use of optical elements in rod lens endoscopes is well known. Rod lens endoscopes typically include two identical rod lenses in their rod lens system and combine the rod lenses symmetrically to relay the image from the objective at a magnification of −1.
A series of these relay pairs convey an image from the objective to an internal image plane. For a right side up image when viewed with an eyepiece, the image is relayed an odd number of times. Therefore, the number of rod lenses in a rod lens system is calculated by using the equation 2+4n where n is an integer (e.g., 2, 6, 10, 14, etc.).
In symmetric rod lens systems, coma, distortion, and lateral color are automatically corrected for each relay pair. Spherical aberration and axial chromatic aberration are corrected by design of the rod lens systems. Astigmatism and field curvature, however, are not corrected, and accumulate with each additional relay. They are generally compensated for in the objective.
To correct for axial chromatic aberration and spherical aberration, each rod lens must have at least two components. Each rod lens is typically a doublet or a triplet. A doublet is a type of lens made up of two simple lenses paired together. A triplet is a type of lens made up of three simple lenses paired together.
Prior art systems require that the rod lenses are doublets to work and that there is at least one correction element or correction lens group in each relay lens system, thus having multiple correction lens groups through the entire optical system. For example, U.S. Pat. No. 7,724,430 to Kasai discloses a rigid endoscope and image transfer optical system having relay lens systems (Re1 to Re7) each having a correction lens group in each of the relay lens systems. Each of Re1 to Re 7 has a triplet positive lens located between each of the rod lenses in the relay lens system. U.S. Pat. No. 7,515,335 to Tomioka discloses endoscope relay lens for a rigid endoscope where the inner end of the lens unit is a cemented doublet, thus having two cemented doublets per relay lens system.
U.S. Pat. Nos. 5,933,275 and 6,163,401 to Igarashi disclose an optical system for non-flexible endoscopes having a plurality of relay lens units, the relay lens units having two bioconvex cemented doublets. U.S. Pat. No. 4,993,817 to Hoogland discloses an endoscope relay optics design having a pair of doublet lens assemblies for incorporation into an optical transfer system. U.S. Pat. No. 4,988,172 to Kanamori et al. discloses an optical system for endoscopes having a relay lens having an achromatic doublet lens consisting of a common glass lens. U.S. Pat. No. 4,693,568 to Takahashi discloses an image transmission optical system for an endoscope comprising two rod-like biconvex lenses and two thick meniscus lenses arranged between the two rod-like biconvex lenses. All of these prior art references disclose a high number of optical elements in each relay lens system. It is disadvantageous to have a high number of optical elements in each relay lens system.
Thus, it is desirable to decrease the number of optical elements in each relay lens system and also to decrease the number of optical elements in the overall endoscope as a whole. Decreasing the number of optical elements reduces the overall cost of the optical system because each additional lens has a cost and decreasing the overall number of optical elements in the system decreases the cost of the system.
Thus, it is desirable to provide an optical system where the spherical aberration and axial chromatic aberration in the entire series of rod lenses is corrected by a doublet or a triplet located between the rod lenses in one or more of the relays, but not in each relay lens system. This allows for the number of optical elements to be reduced, while still allowing for the axial chromatic aberration and spherical aberration to be corrected.